Reflecting light with a certain angle of incidence can change the polarization state of the reflected light relative to the incident light, and the total reflectance is dependent on the polarization state. As shown in FIG. 1, a beam of natural light is polarized after being reflected.
The plane of incidence is determined by the incident light and the normal to the reflecting surface. A beam of incident light can be divided into two perpendicular-polarized waves, i.e. s- and p-wave, wherein the oscillation direction of s-wave is perpendicular to the incident plane and the oscillation direction of p-wave is parallel to the incident plane. After reflection, the light intensity isIm=Ip·ρp(θ)+Is·ρs(θ)
As shown in FIG. 2, the difference between the reflectance of s-wave (RS) and p-wave (Rp) causes the reflected light to have unwanted polarization. For example, if the incident light is natural light, the reflected light will be partially polarized light, and if the incident light is polarized light, the polarization state of reflected light will be changed. In addition, the difference between s-wave reflectance and p-wave reflectance is dependent on the angle of incidence. Therefore, the unwanted polarization is dependent on the angle of incidence.
The total reflectance is the ratio of the reflected light intensity to the incident light intensity. Because the natural light intensity of s-wave or p-wave is half of the total light intensity, the total reflectance of the natural light is the arithmetic mean value of the s-wave reflectance and p-wave reflectance. For polarized incident light, the total reflectance is more complicated to determine due to the polarization dependence. Under a certain angle of incidence, the total reflectance is dependent on the polarization state of the incident light. When the angle of incidence is relatively big, this dependence can result in an inevitable principal error in accurate optical measurement.
In addition, according to FIG. 2, the polarization caused by reflection on the medium surface is different from the polarization caused by reflection on the metal surface. The average reflectance on metal surface is ordinarily higher than that on the medium surface. Furthermore, the average reflectance and the difference between s-wave reflectance and p-wave reflectance are changed according to the different metal material.
Reflector is a very important optical element, and it is widely used in varieties of fields of optics. Although polarization is an unnegligible problem in many fields, there is no convenience, economical, and effective way to eliminate unwanted polarization in reflection.
At present, the ordinary method to solve this problem is to coat on reflector surface, but this method is expensive and the effect is limited. This method can reduce the unwanted polarization at some degree. However, the difference between p-wave reflectance and s-wave reflectance still exists in the. Furthermore, the coating method can only be applied to the reflection with a narrow wave band and with limited angle of incidence. The angle of incidence can not be freely changed. Otherwise, the effect of reducing polarization will be affected.
This invention can conveniently and effectively eliminate the polarization by reflection, and can be applied in a microscope or a goniophotometer or other optical systems with reflection in the light path.